The Collaborative International Dictionary of English v.0.48:

Guard \Guard\, n. [OF. guarde, F. garde; of German origin; cf.
   OHG. wart, warto, one who watches, warta a watching, Goth.
   wardja watchman. See Guard, v. t.]
   [1913 Webster]
   1. One who, or that which, guards from injury, danger,
      exposure, or attack; defense; protection.
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            His greatness was no guard to bar heaven's shaft.
                                                  --Shak.
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   2. A man, or body of men, stationed to protect or control a
      person or position; a watch; a sentinel.
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            The guard which kept the door of the king's house.
                                                  --Kings xiv.
                                                  27.
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   3. One who has charge of a mail coach or a railway train; a
      conductor. [Eng.]
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   4. Any fixture or attachment designed to protect or secure
      against injury, soiling, or defacement, theft or loss; as:
      (a) That part of a sword hilt which protects the hand.
      (b) Ornamental lace or hem protecting the edge of a
          garment.
      (c) A chain or cord for fastening a watch to one's person
          or dress.
      (d) A fence or rail to prevent falling from the deck of a
          vessel.
      (e) An extension of the deck of a vessel beyond the hull;
          esp., in side-wheel steam vessels, the framework of
          strong timbers, which curves out on each side beyond
          the paddle wheel, and protects it and the shaft
          against collision.
      (f) A plate of metal, beneath the stock, or the lock
          frame, of a gun or pistol, having a loop, called a
          bow, to protect the trigger.
      (g) (Bookbinding) An interleaved strip at the back, as in
          a scrap book, to guard against its breaking when
          filled.
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   5. A posture of defense in fencing, and in bayonet and saber
      exercise.
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   6. An expression or admission intended to secure against
      objections or censure.
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            They have expressed themselves with as few guards
            and restrictions as I.                --Atterbury.
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   7. Watch; heed; care; attention; as, to keep guard.
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   8. (Zool.) The fibrous sheath which covers the phragmacone of
      the Belemnites.
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   Note: Guard is often used adjectively or in combination; as,
         guard boat or guardboat; guardroom or guard room; guard
         duty.
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   Advanced guard, Coast guard, etc. See under Advanced,
      Coast, etc.

   Grand guard (Mil.), one of the posts of the second line
      belonging to a system of advance posts of an army.
      --Mahan.

   Guard boat.
      (a) A boat appointed to row the rounds among ships of war
          in a harbor, to see that their officers keep a good
          lookout.
      (b) A boat used by harbor authorities to enforce the
          observance of quarantine regulations.

   Guard cells (Bot.), the bordering cells of stomates; they
      are crescent-shaped and contain chlorophyll.

   Guard chamber, a guardroom.

   Guard detail (Mil.), men from a company regiment etc.,
      detailed for guard duty.

   Guard duty (Mil.), the duty of watching patrolling, etc.,
      performed by a sentinel or sentinels.

   Guard lock (Engin.), a tide lock at the mouth of a dock or
      basin.

   Guard of honor (Mil.), a guard appointed to receive or to
      accompany eminent persons.

   Guard rail (Railroads), a rail placed on the inside of a
      main rail, on bridges, at switches, etc., as a safeguard
      against derailment.

   Guard ship, a war vessel appointed to superintend the
      marine affairs in a harbor, and also, in the English
      service, to receive seamen till they can be distributed
      among their respective ships.

   Life guard (Mil.), a body of select troops attending the
      person of a prince or high officer.

   Off one's guard, in a careless state; inattentive;
      unsuspicious of danger.

   On guard, serving in the capacity of a guard; doing duty as
      a guard or sentinel; watching.

   On one's guard, in a watchful state; alert; vigilant.

   To mount guard (Mil.), to go on duty as a guard or
      sentinel.

   To run the guard, to pass the watch or sentinel without
      leave.

   Syn: Defense; shield; protection; safeguard; convoy; escort;
        care; attention; watch; heed.
        [1913 Webster]

WordNet (r) 3.0 (2006):

guard of honor
    n 1: an escort for a distinguished guest or for the casket at a
         military funeral [syn: honor guard, guard of honor]